In digital modulation which requires suppression of a DC component, such as RLL (Run Length Limited) or the like, when recording data onto a recording medium, the DC component of a signal needs to be suppressed so that a modulated signal can be stably reproduced. FIG. 17 illustrates a structure of a conventional digital modulation apparatus which is used as an optical disc apparatus capable of signal recording and the like, particularly a communication apparatus of performing optical communication. The conventional digital modulation apparatus 10 comprises a modulated code generator 11 of receiving source data DATA and generating modulated codes CODEa and CODEb, two DSV calculators 12 of receiving the modulated codes CODEa and CODEb and calculating DSVa and DSVb, respectively, a modulated code determinator 13 of comparing sizes of DSVa and DSVb to determine a modulated code to be selected, and a modulated code selector 14 of selecting one of the codes CODEa and CODEb based on a result of determination by the modulated code determinator 13 and outputting the selected code as a modulated code CODE. In this manner, the conventional digital modulation apparatus 10 selects an optimal modulated code from a plurality of modulated codes using a DSV (Digital Sum Value) of a binary signal as a measure so that the DSV consistently has a minimum absolute value.
The DSV is a measure which has been introduced so as to take a balance of the amounts of “0” and “1” in an NRZI (Non Return to Zero Invert) format modulated signal to be transmitted through a transmission channel, and is obtained by accumulating “−1” or “+1” if each channel bit constituting the NRZI format modulated signal is “0” or “1”, respectively. In other words, if the DSV is “0”, the channel bits constituting the NRZI format modulated signal from a modulation starting point to a current time include the same number of “0”s and “1”s. Therefore, by performing a code selection control (also referred to as a DC control) so that the DSV value approaches “0”, the DC component of the modulated signal can be suppressed to a small value (see, for example, Patent Publication 1).
A method of generating a modulated data sequence while inserting a binding bit between each code has been proposed (see, for example, Non-patent Publication 1). Also in this case, if the modulated code is considered to be composed of a code portion (14 bits) and a binding bit portion (3 bits), this method can be said to be of the above-described digital modulation scheme using the DC control method using a DSV as a measure.
(Patent Publication 1) JP H09-162744 A (pages 5 to 9, FIGS. 1 and 2)
(Non-patent Publication 1) Heitaro Nakajima and Hiroshi Ogawa, “Zukai Konpakutodhishuku Dokuhon (Illustrated Compact Disc)”, revised 2nd version, Ohmsha, December 1993, pp. 125-131